The objective of our research program is to increase understanding of the general mechanism of gene activation in terms of chromatin structure. Our working hypothesis is that the association of different major proteins with given loci (and perhaps the modification of associated proteins) will have a significant effect on the accessibility of loci to transcription. We will continue to focus on the major nonhistone chromosomal proteins (NHC proteins) of Drosophila, studying in particular their patterns of in situ distribution on polytene chromosomes, using an immunofluorescent assay. NHC proteins for study are being isolated from chromomatin both by chemical fractionation techniques and by techniques designed to exploit the biological associations of the proteins (binding to the DNA: histone complex, release using DNase I, etc.). The structure of the chromatin at the heat shock loci before and after activation is being investigated by nuclease digestion studies. Concurrently work is being undertaken to develop a reliable in vitro transcription assay using Drosophila nuclei and chromatin. Ultimately one hopes to propose a model of chromatin structure for the general active and inactive forms which can be tested both by confirming the distribution of the major protein components in vivo and by examining the properties of the complex in vitro.